[PD] Raspberry Pi does denormals

[katja]

Miller, the vanilla Pd which can be installed from Raspbian with
apt-get or Synaptic does have the subnormals problem, as can be
checked with a test patch attached with my first post. When an input
signal to [lop~] is shut off, CPU load increases substantially. Output
values go down in the order of 1e-44, subnormal range. I was working
on reverb algo's showing the same problem, and compiled with option
-ffastmath / --fast-math to see if that would turn on RunFast mode,
but it didn't.

I'm not familiar with ARM and it's coprocessors, but from Intel I do
know that gcc doesn't implement certain specified optimization options
(notably SSE versions) unless you also mention a processor type that
can handle it . A similar case could be with Rpi's vfpv2; it can do
RunFast mode but gcc doesn't implement it, until you find a way to
specify vfpv2 (vfpv1 can't do RunFast). Miller, if you succeeded in
getting automatic flush-to-zero on the Pi, it may be related to other
flags which you've set. Arch flags which I've set so far are
-march=armv6 and -mfpu=vfp. Option -mfpu=vfpv2 is not allowed. I would
be happy to do further testing with compiler options, if you know
some. The big-or-small checks are rather expensive for RPi, that's
what I've found.

Katja


On Sun, Jan 20, 2013 at 8:24 PM, Miller Puckette <msp at ucsd.edu> wrote:
> Hi all...
>
> I think it's possible to get flush-to-zero behavior on the Pi (ARMv6) by
> calling gcc with --fast-math.  At any rate what I found was that, if I
> compiled without --fast-math, when numbers got small (e.g., when a
> reverberator decays down past 10^-38 or so), the patch would suddenly jump
> in CPI usage as if it were trappnig to the kernel (as it does for i386).
> But when I added --fast-math the problem went away.
>
> On i386 and x86_64, I believe that one can't get flush-to-zero (at least in
> the "normal" non-SSE floating point instructions) so there's no choice but
> to use a macro such as PD_BADFLOAT to protect against that.  So in m_pd.h the
> PD_BADFLOAT macro is only "turned on" for Intel.
>
> However I've been mistaken many times about all this in the past and won't
> be surprised if I'm mistaken again.
>
> cheers
> Miller
>
> On Sun, Jan 20, 2013 at 11:12:28AM -0500, Hans-Christoph Steiner wrote:
>>
>> I think this is what you want, from 'man gcc'.  Its interesting to note that
>> the NEON mode, which provides SIMD, also does not do denormals:
>>
>> -mfpu=name
>> -mfpe=number
>> -mfp=number
>>     This specifies what floating point hardware (or hardware emulation) is
>>     available on the target.  Permissible names are: fpa, fpe2, fpe3, maverick,
>>     vfp, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16,
>>     neon, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16 and neon-vfpv4.  -mfp and
>>     -mfpe are synonyms for -mfpu=fpenumber, for compatibility with older
>>     versions of GCC.
>>
>>     If -msoft-float is specified this specifies the format of floating point
>>     values.
>>
>>     If the selected floating-point hardware includes the NEON extension (e.g.
>>     -mfpu=neon), note that floating-point operations will not be used by GCC's
>>     auto-vectorization pass unless -funsafe-math-optimizations is also
>>     specified.  This is because NEON hardware does not fully implement the IEEE
>>     754 standard for floating-point arithmetic (in particular denormal values
>>     are treated as zero), so the use of NEON instructions may lead to a loss of
>>     precision.
>>
>>
>> .hc
>>
>> On 01/20/2013 06:54 AM, katja wrote:
>> > I was assuming, or maybe just hoping? that Raspberry Pi (and ARM
>> > devices in general) would not suffer from Denormal's disease like
>> > Intel processors do. But guess what: Pi's float coprocessor is IEEE
>> > 754 compliant and does all denormals by default (can check with
>> > attached denorm-test.pd). Bummer! As if one would use an ARM device to
>> > calculate the size of a Majorana particle, rather than doing simple
>> > dsp. Do we really need to enable PD-BIGORSMALL() checks for this poor
>> > little processor? There seems to be something called 'RunFast mode'
>> > for Pi's float processor vfpv2, but I see no way how to enable this
>> > via gcc. Option -ffast-math is allowed but doesn't do the trick. Can't
>> > find an option to set vfpv2 specifically, in gcc docs.
>> >
>> > Katja
>> >
>> >
>> >
>> > _______________________________________________
>> > Pd-list at iem.at mailing list
>> > UNSUBSCRIBE and account-management -> http://lists.puredata.info/listinfo/pd-list
>> >
>>
>> _______________________________________________
>> Pd-list at iem.at mailing list
>> UNSUBSCRIBE and account-management -> http://lists.puredata.info/listinfo/pd-list
>
> _______________________________________________
> Pd-list at iem.at mailing list
> UNSUBSCRIBE and account-management -> http://lists.puredata.info/listinfo/pd-list